This invention relates to an improved magnetic circuit for an electromagnetic bias coil used in a magneto-optical recording disk apparatus.
In an erasable magneto-optical recording disk apparatus, data that is recorded on a perpendicular magnetic anisotropic disk can be erased and data can be written thereon. In addition, data recording capacity of this apparatus is higher than that of the conventional magnetic data recording apparatus such as a magnetic disk. Because of their high storage capacity, magneto-optical recording disk apparatus have been the subject of extensive study and development.
A typical prior art erasable magneto-optical recording disk apparatus is schematically illustrated in FIG. 1, wherein parts having no relation to the explanation of the invention are not shown. In FIG. 1, numeral 1 denotes a recording media comprising a film of a perpendicular magnetic anisotropic material, such as TeFeCo (tellurium-iron-cobalt). A light 2 emitted from a semiconductor laser 2' is reflected by a beam splitter 3 and then focused by a lens system 4 onto a recording element 1-1 ("recording bit") in the recording media 1. A magnetic bias field is applied perpendicular to the recording media 1 by applying a current through an electromagnetic bias coil 5. The light beam 2 is radiated onto the recording bit until the recording bit is heated as high as approximately the Curie point of the material. At this temperature, the direction of the magnetic field in magnetic anisotropic material is oriented in the same direction as the applied magnetic bias field, in the area of the recording bit. This orientation of the recording bit's magnetic field remains when the recording bit is cooled after removing the light 2 therefrom.
In short, continuously radiating light onto the recording bit with a constant magnetic bias field applied thereto causes the recording media 1 be uniformly magnetized. Thus, previously recorded data can be erased. Writing data in an erased recording media 1 can be carried out as follows. The direction of the current applied to the electromagnetic coil 5 is reverse from that used during erase, and maintained. In other words, the magnetic bias field is reversed. Then, the light source 2' illuminates the desired recording bit. This heats the recording bit, permitting its magnetization direction to be changed.
Reading data stored in the recording bit can be carried out as follows. The light beam 2, which must be weak enough not to permit erasure of the recorded data during the reading process, illuminates a recording bit to be read out. No magnetic field is applied at this time. The polarization angle of the light reflected from the recording bit varies depending on the direction of its magnetization. This phenomena is known as a Kerr effect. Accordingly, detecting the polarization of the reflected light allows detection of the recorded digital data.
The light source 2', the optical system 4 for focusing the light and the coil 5 are installed on a carriage 6 that is movable in order to roughly trace a track of the disk. A tracking servo mechanism is included in the optical system (e.g. 2, 2', 3, 4 and 6) in order to precisely trace a particular track. This mechanism is not shown in order to simplify the figures.
FIG. 2 is a schematic, cross-sectional view of a configuration for a recording media (disk) 1, an optical system 4 and a bias coil 5 which does not have either a yoke or a core. The recording media 1 comprises a recording layer 11 including a perpendicular magnetic anisotropic material, and a protection layer 12 including a transparent material, such as glass. The optical system 4 comprises an outer cylinder 41 and a plurality of lenses 42. A laser light 21 emitted from the light source 2' is focused onto the recording bit 1-1 in the recording layer 11. The bias coil 5 can be a solenoid coil wound in a shape of cylinder, and must provide a magnetic field intensity of, for example, 300 Oe (Oersted) in a direction perpendicular to the recording layer 11.
FIG. 3 schematically illustrates a magnetic flux 8 provided by the bias coil 5. The protection layer 12 is typically as thick as 1.2 mm, and an air gap between the protection layer 12 and an end of the bias coil 5 is typically 1 mm so as to provide a margin for thermal deformation, etc. Because of these distances, a large current must be used in the coil in order to produce the required magnetic field intensity at the recording bit. However, large currents cause the temperature of coil 5 to rise. Such a temperature rise is a limitation of prior disk apparatus.
In order to avoid the temperature/current requirement problem, several structures have been proposed. For example, Tanaka in Japanese unexamined patent publication Sho 60-29904 discloses a magnetic core in the inner diameter of the coil (FIG. 5) and a yoke around the coil, with a magnetic core for the coil (FIG. 6).
Okada, in Japanese unexamined patent publication Sho 61-32244, and Shinbara in Japanese unexamined patent publication Sho 60-117403 propose using an additional magnetization device on the opposite side of the recording disk 1. However, such configurations block installation of the optical system. These configurations, therefore, do not allow both surfaces of a disk to be used in order to increase the recording capacity of a single disk.
The physical space that a coil and a yoke can occupy is limited by other peripheral devices. Typical dimensions of the space are: for example, 27 mm outer diameter; and 12 mm inner diameter (see, e.g., FIG. 4-6). Accordingly, the space occupied by the yoke or by the core reduces the space available for the coil. One way to reducing the size of the coil is to change the number of turns of the coil. This changes the electrical resistance of the coil, the magnetic field intensity at the recording bit, power consumption of the coil, and magnetic field intensity per power consumption. These parameters are shown in Table 1. The data is shown therein for the condition that the recording bit is located 2.2 mm from the coil's flat end facing the disk as represented in FIGS. 4-6.
It is usual to provide a recording layer on both surfaces of a disk in order to increase the recording capacity of the disk. The requires that two sets of coils such as shown in FIGS. 4, 5 or 6 and optical systems 4 in FIG. 1 be provided on opposing sides of the disk surface. Table 2 shows the number of turns of the coil, the electrical resistance of the coil, the magnetic field intensity at the recording bit, power consumption of the coil, and magnetic field intensity per consumption power for such a structure. FIGS. 8-10 respectively illustrate the magnetic field intensity generated when a constant current of 0.25 A is applied to the coils shown in FIGS. 4-6. In FIGS. 8-10, the horizontal axis represents a distance from the recording bit along the disk surface. In FIGS. 8-10, the solid lines relate to the left hand side scale and represent the magnetic field intensity of single coil; and the dotted lines relate to the right hand side scale and represent the magnetic field intensity of two coils together with the associated core and yoke, if any.
From FIGS. 9 and 10 respectively, it is seen that with the coil of FIG. 5 or 6 (where the magnetic core is provided along the inner diameter of the coil), the magnetic field intensity at the recording bit is less than the magnetic field intensity in a region of the disk surface above the core. The actual magnetic field intensity and the overall shape of the magnetic field distribution curve varies depending on the shape of the coil, the yoke, the thickness of the protection coating and the air gap. The coil parameters used to achieve the required 300 Oe by increasing coil current over that shown in Tables 1 and 2 are shown in Table 3 for a single coil and in Table 4 for two coils. From these tables, it is apparent that the magnetic field intensity at the recording bit cannot be easily increased without undesirable temperature increases in the coil.